This invention relates in general to equipment for compressing gas, and in particular to a system for compressing gas from a low pressure source into a storage vessel at a higher pressure.
Compressed natural gas is used for supplying fuel for vehicles as well as for heating and other purposes. The gas is stored by the user in a tank at initial pressure of about 3,000 to 5,000 psi., typically 3600 psi. When the compressed natural gas is substantially depleted, the user proceeds to a dispensing station where compressed natural gas is stored in large dispensing tanks at pressures from 3,000 to 5,000 psi. The dispensing station refills the user""s tank from its dispensing tank.
If the station is located near a gas pipeline, when the station""s storage vessels become depleted, they can be refilled from the natural gas pipeline. For safety purposes, the pipeline would be at a much lower pressure, such as about 5 to 100 psi. This requires a compressor to fill the dispensing tank by compressing the gas from the gas source into the dispensing tank. Compressors are typically rotary piston types. They require several stages to compress gas from the low to the high pressure used for natural gas vehicle applications. These compressors generate significant amounts of heat which must be dissipated in inner cooling systems between the compression stages. These compressors may be expensive to maintain.
Also, in certain parts of the world, natural gas pipelines are not readily available. The dispensing stations in areas far from a pipeline or gas field rely on trucks to transport replacement dispensing tanks that have been filled by a compressor system at a pipeline. The same compressors are used at the pipeline to fill the dispensing tanks.
Hydraulic fluid pumps are used in some instances to deliver hydraulic fluid under pressure to a tank that contains gas under pressure. A floating piston separates the hydraulic fluid from the gas. The hydraulic fluid maintains the pressure of the gas to avoid a large pressure drop as the gas is being dispensed.
In this invention, gas is compressed from a gas source into a storage tank by an apparatus other than a conventional compressor. In this method, a first tank assembly is filled with gas from the gas source. Hydraulic fluid is drawn from a reservoir and pumped into the first tank assembly into physical contact with the gas contained therein. This causes the gas in the first tank assembly to flow into the storage reservoir as the first tank assembly fills with hydraulic fluid. The second tank assembly, which was previously filled with hydraulic fluid, simultaneously causes the hydraulic fluid within it to flow into a reservoir. The hydraulic fluid is in direct contact with the gas as there are no pistons that seal between the hydraulic fluid and the gas.
When the first tank assembly is substantially filled with hydraulic fluid and the second tank assembly substantially emptied of hydraulic fluid, a valve switches the sequence. The hydraulic fluid flows out of the first tank assembly while gas is being drawn in, and hydraulic fluid is pumped into the second tank assembly, pushing gas out into the storage vessel. This cycle is repeated until the storage vessel reaches a desired pressure.